1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor thin film, and particularly to a method of manufacturing a semiconductor device constituted by a thin film transistor (TFT) using a crystalline silicon film.
In the present specification, the semiconductor device means any device functioning by using a semiconductor, and not only a simple component such as a TFT but also an electrooptical device, a semiconductor circuit, and an electronic equipment provided with those are contained in the semiconductor device.
2. Description of the Related Art
In recent years, a TFT used for an electrooptical device such as an active matrix type liquid crystal display device has been actively developed.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. Moreover, a system-on-panel having built-in logic circuits, such as a γ correction circuit, a memory circuit, and a clock generation circuit, has been also developed.
Since such a driver circuit and a logic circuit are required to perform high speed operation, it is improper to use an amorphous silicon film as an active layer. Thus, in the present circumstances, a TFT using a crystalline silicon film (polysilicon film) as an active layer has become the mainstream.
The present inventors et al. disclose the technique for obtaining a crystalline silicon film on a glass substrate in Japanese Patent Laid-Open 8-78329. In the technique disclosed in the publication, a catalytic element for facilitating crystallization is selectively added in an amorphous silicon film, and a heat treatment is carried out to form a crystalline silicon film extending from an added region.
According to this technique, a crystallizing temperature of an amorphous silicon film can be lowered by 50 to 100° C. by action of the catalytic element, and a time required for crystallization can also be reduced to ⅕ to 1/10. Since the crystallization of the silicon film progresses almost parallel to the surface of a substrate toward a lateral direction, the present inventors et al. refer to this crystallized region as a lateral growth region.
Since the catalytic element is not directly added in the lateral growth region, the region has a feature that the concentration of the catalytic element remaining in the film is low as compared with the case where the catalytic element is directly added. For example, although the content of the catalytic element is in the order of 1019 in the case of direct addition, the content in the lateral growth region is in the order of 1018 which is smaller by one figure.
Although it becomes possible to obtain a silicon film having excellent crystallinity by the above crystallizing technique at a relatively low temperature, since the catalytic element is included in the film, the control of the amount of the addition is subtle, and a problem has occurred in the reproducibility and stability (stability in electrical characteristics of a device obtained).
Besides, there has also occurred a problem that change of characteristics of the obtained semiconductor device with the lapse of time and the dispersion of OFF values (off current) as characteristic values of the TFT are large by the influence of the catalytic element remaining in the film.